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Frequently Asked Questions...

I need information about solar electricity generation?

I have made a project on solar electricity power generation and expected to say something about it ..I need your help for the following informations.
(1) How the whole process work?.(asking about the solar energy receptor)
(2) How the sun rays are converted in to electrical charges?
(3)What are the good and bad effects of solar power generation?
and so on..

I have search a number of site to know about this but didn't get satisfying answers.I have only two days in my hand to speak about this so please give some information,no links please.As loking for the above information in thos links may take time and donot have much time.
In other word i am asking you people to write a lecture/essay for me on Solar electricity/power generation.
Hope you people would answer me satisfactorily.

Correction: This article incorrectly identifies the phenomenon in which visible light induces a current in an electrical circuit as the photoelectric effect studied by Albert Einstein. It is a separate phenomenon. In addition, the article incorrectly states that when a semiconductor is doped with impurities, it acquires an overall charge. Doping only alters the distribution of electrons, but this suffices to produce an electric current when a sandwich of semiconductors is exposed to light.

Is PV Right for Your Home?

The size and layout of a PV system depend on your needs, budget, and location. Some factors to consider when choosing a system include:

Cost-benefit ratio. PV systems are still fairly expensive—a typical two-kilowatt residential system can cost $20,000 or more. However, a variety of financial incentives such as rebates and tax breaks are available that can decrease your costs by 50 percent or more. To find out what incentives are available in your area, visit the online Database of State Incentives for Renewable Energy at www.dsireusa.org. You should also keep in mind that most PV systems last 20 years or longer, so in addition to the environmental benefits the technology provides, its high up-front costs will translate into lower electricity bills for the life of the system.

Electricity demand. PV systems can meet all or part of your home’s electricity needs, and can be expanded over time. Along with the cost, the amount of space you have available for PV panels may be a deciding factor in how large a system you can install. Conservation and energy efficiency can lower your household electricity demand, allowing you to reduce your initial investment in PV panels.

Orientation. To absorb the most sunlight possible, PV arrays need to be mounted on a south-facing surface (often a roof) that receives direct sunlight. The exact orientation and tilt angle of the PV array depends on whether you want to maximize electricity generation over the course of an entire year or only during the summer, and whether, on a day-to-day basis, you want to optimize overall power or the power needed during peak electricity demand.

Last year, the solar photovoltaic (PV) industry celebrated its 50th anniversary, but the technology was under development long before 1954. In 1839, French physicist Edmund Bequerel discovered that certain materials produced sparks when exposed to light, and Albert Einstein studied this “photoelectric effect” in a 1905 paper that later won him the Nobel Prize. It was Bell Laboratories’ 1954 introduction of a small solar battery, however, that is considered the official birth of the PV industry. Today this technology is used to provide electricity for wristwatches and calculators, roadside call boxes and water pumps, homes and businesses, and even Earth-orbiting satellites.

Turning Sunlight into Electricity

The most important components of a PV cell are two layers of semiconductor material generally composed of silicon crystals. On its own, crystallized silicon is not a very good conductor of electricity, but when impurities are intentionally added—a process called “doping”—the stage is set for creating an electric current. The bottom layer of the PV cell is usually doped with boron, which bonds with the silicon to produce an overall positive charge (P). The top layer is doped with phosphorus, which bonds with the silicon to produce a negative charge (N).

The surface between the resulting “p-type” and “n-type” semiconductors is called the P-N junction (see the diagram above). Electron movement at this surface produces an electric field that only allows electrons to flow from the p-type layer to the n-type layer.

When sunlight enters the cell its energy knocks electrons loose in both layers. Because of the opposite charges of the layers, the electrons want to flow from the n-type layer to the p-type layer, but the electric field at the P-N junction prevents this from happening. The presence of an external circuit, however, provides the necessary path for electrons in the n-type layer to travel to the p-type layer. Extremely thin wires running along the top of the n-type layer provide this external circuit, and the electrons flowing through this circuit provide the cell’s owner with a supply of electricity.

Because silicon tends to be shiny and reflect light, the top (n-type) layer is usually given a thin anti-reflective coating to make sure as much sunlight gets into the cell as possible. Even with this assistance, a PV cell is only able to convert a fraction of the sun’s energy into electricity—typically 11 to 16 percent depending on the materials used and the cell type—but this is still enough energy for many residential and commercial applications.

Most PV systems consist of individual square cells averaging about four inches on a side. Alone, each cell generates very little power (less than two watts), so they are often grouped together as modules. Modules can then be grouped into larger panels encased in glass or plastic to provide protection from the weather, and these panels, in turn, are either used as separate units or grouped into even larger arrays.

A Solution for Everyone

You don’t have to live in the desert to reap the benefits of solar power—PV systems can provide electricity no matter where you live. Because PV systems can only generate electricity when the sun is not obscured, however, it is usually necessary to have some way of supplementing or storing the power.

One option is to connect to the national grid, which not only allows you to transfer any excess power your system generates to the grid—causing your electric meter to run backward—but also to draw electricity from the grid during extended periods of cloudy weather. A disadvantage of being connected to the grid is that if there is a blackout, the equipment installed by your local utility to keep your power “in phase” with the grid will shut down your PV system.

For a PV system that is not connected to the grid, batteries are available that can store enough energy to provide power for up to several days. This option increases your level of self-sufficiency, but adds to the cost of the system, requires monitoring of cable connections and fluid levels, and poses an environmental hazard at the time of disposal.

The PV industry continues to research new materials and designs in order to bring system costs down while improving versatility and efficiency. Thin-film PV technology, for example, is now available to be integrated into rooftop shingles, and nanotechnology is not far from providing solar batteries for cell phones and laptop computers. As long as the sun shines, photovoltaics will be an important step toward a cleaner energy future.

To learn more about PV technology, visit the U.S. Department of Energy website at www.eere.energy.gov/solar

Kristen Graf is an assistant in the Clean Energy Program. Jeff Deyette is an analyst in the program.